In recent years, it has become common to transmit large-volume data, such as still image data and moving image data in addition to audio data in cellular mobile communication systems, in response to spread of multimedia information. Active studies associated with techniques for achieving a high transmission rate in a high-frequency radio band have been conducted to achieve large-volume data transmission.
When a high frequency radio band is utilized, however, attenuation increases as the transmission distance increases, although a higher transmission rate can be expected within a short range. Accordingly, the coverage area of a radio communication base station apparatus (hereinafter, abbreviated as “base station”) decreases when a mobile communication system using a high frequency radio band is actually put into operation. Thus, more base stations need to be installed in this case. The installation of base stations involves reasonable costs, however. For this reason, there has been a high demand for a technique that provides a communication service using a high-frequency radio band, while limiting an increase in the number of base stations.
In order to meet such a demand, studies have been carried out on a relay technique in which a radio communication relay station apparatus (hereinafter, abbreviated as “relay station”) is installed between a base station and a radio communication mobile station apparatus (hereinafter, abbreviated as “mobile station”) to perform communication between the base station and mobile station via the relay station for the purpose of increasing the coverage area of each base station. The use of relay technique allows a mobile station not capable of directly communicating with a base station to communicate with the base station via a relay station.
An LTE-A (long-term evolution advanced) system for which the introduction of the relay technique described above has been studied is required to maintain compatibility with LTE (long term evolution) in terms of a smooth transition from and coexistence with LTE. For this reason, mutual compatibility with LTE is required for the relay technique as well.
FIG. 1 illustrates example frames in which control signals and data are assigned in the LTE system and the LTE-A system.
In the LTE system, a DL (downlink) control signal from a base station to a mobile station is transmitted through a DL control channel, such as a PDCCH (physical downlink control channel). In LTE, a DL grant indicating DL data assignment and a UL (uplink) grant indicating UL data assignment are transmitted through a PDCCH. A DL grant reports that a resource in the subframe in which the DL grant is transmitted has been allocated to the mobile station. Meanwhile, in an FDD system, a UL grant reports that a resource in the fourth subframe after the subframe in which the UL grant is transmitted has been allocated to the mobile station. In a TDD system, UL grant reports that the resource in a subframe transmitted after four or more subframes from the subframe in which the UL grant is transmitted has been allocated to the mobile station. In the TDD system, the subframe to be assigned to the mobile station, or the number of subframes before the assigned subframe in which the UL grant is transmitted is determined in accordance with the time-division pattern of the UL and DL (hereinafter referred to as “UL/DL configuration pattern”). Regardless of the UL/DL configuration pattern, the UL subframe is a subframe after at least four subframes from the subframe in which the UL grant is transmitted.
In the LTE-A system, relay stations, in addition to base stations, also transmit control signals to mobile stations in PDCCH regions in the top parts of subframes. With reference to a relay station, DL control signals have to be transmitted to a mobile station. Thus, the relay station switches the processing to reception processing after transmitting the control signals to the mobile station to prepare for receiving signals transmitted from the base station. The base station, however, transmits a DL control signal to the relay station at the time the relay station transmits the DL control signal to the mobile station. The relay station therefore cannot receive the DL control signal transmitted from the base station. In order to avoid such inconvenience in the LTE-A, studies have been carried out on providing a region in which downlink control signals for relay stations are located (i.e., relay PDCCH (R-PDCCH) region) in a data region. Similar to the PDCCH, placing a DL grant and UL grant on the R-PDCCH is studied. In the R-PDCCH, as illustrated in FIG. 1, placing a DL grant in the first slot and a UL grant in the second slot is studied (refer to Non-patent Literature 1). Placing the DL grant in the first slot reduces a delay in decoding the DL grant, and allows relay stations to prepare for ACK/NACK transmission for DL data (transmitted in the fourth subframe following reception of a DL grant in FDD). Each relay station finds the downlink control signal intended for the relay station by performing blind-decoding on downlink control signals transmitted using R-PDCCH from a base station within a resource region indicated using higher layer signaling from the base station (i.e., search space).
As described above, the base station notifies the relay station of the search space corresponding to the R-PDCCH by higher layer signaling. Notification of the search space corresponding to the R-PDCCH may be performed in two different ways: (1) notification using a PRB (physical resource block) pair as a single unit; or (2) notification using an RBG (resource block group) as a single unit. The term, “PRB (physical resource block) pair” refers to a set of PRBs in the first and second slots, whereas the term, “PRB” refers to an individual PRB in either the first or second slot. Hereinafter, a PRB pair may simply be referred to as “PRB.” A resource block group (RBG) is a unit used when a plurality of PRBs are allocated as a group. The size of an RBG is determined on the basis of the bandwidth of the communication system.
An R-PDCCH has four aggregation levels, i.e., levels 1, 2, 4, and 8 (for example, refer to Non-patent Literature (hereinafter, abbreviated as “NPL” 1). Levels 1, 2, 4, and 8 respectively have six, six, two, and two mapping candidate positions. The term “mapping candidate position” refers to a candidate region to which a control signal is to be mapped. When a single terminal is configured with one aggregation level, control signals are actually mapped to one of the multiple mapping candidate positions of the aggregation level. FIG. 2 illustrates example search spaces corresponding to an R-PDCCH. The ovals represent search spaces at various aggregation levels. The multiple mapping candidate positions in the search spaces at the different aggregation levels are continuous on VRBs (virtual resource blocks). The mapping candidate positions in the VRBs are mapped to PRBs (physical resource blocks) through higher layer signaling.
Furthermore, when the base station transmits a DL grant in the R-PDCCH region directed to the relay station and assigns a PDSCH in RBG units to the relay station, the DL grant and PDSCH may be placed on the same RBG in a given subframe. That is, as shown by the top RBG in FIG. 3, when a DL grant is mapped to region (a) of a given RBG, regions (b) and (c) in the given subframe are allocated to PDSCHs by the DL grant. RBG is formed of M (M is a natural number equal to or greater than two) PRB pairs. Region (a) is in a first PRB pair allocated to the DL grant (i.e., “allocated PRB” pair), in a first slot other than the PDCCH region. Region (b) belongs to a second slot in the allocated PRB pair and is provided as a search space for a UL grant. Region (c) resides among the M PRB pairs, which form the RBG including the allocated PRB pair, in a region excluding the allocated PRB pair and the PDCCH region. As shown on the side of the top RBG in FIG. 3, if the PDSCH is allocated to the RBG, the value of the resource allocation bit (RA bit) for the RBG is “1” in the DL grant.
In a case where DL and UL grants are mapped to the same RBG in a given subframe, the DL grant is mapped to region (a), and the UL grant is mapped to region (b), as shown by RBG in the middle of FIG. 3. Regions (b) and (c) are not allocated to PDSCH. In such a case, zero is assigned to the value of the resource allocation bit included in the DL grant. Thus, a terminal that receives a DL control signal can determine whether the resource allocation as the RBG at the top in FIG. 3 or as the RBG in the middle of FIG. 3 is performed depending on the value, i.e., zero or one, of the resource allocation bit in the DL grant intended for the terminal for each RBG.
In a case where a DL grant and a UL grant are mapped to the same RBG in a given subframe, as shown by the RBG at the bottom in FIG. 3, if the value of the resource allocation bit for the RBG included in the DL grant is assumed to be “1,” the UL grant and the PDSCH that are allocated to the same resource collide with each other. To avoid such collision, the base station sets the value of the resource allocation bit to zero when mapping the UL grant.